arithmetic calculator application

[juliesyumyum]

Hey! I need to program a simple arithmetic calculator. I've done the parser, infix to postfix, etc. Everything's working with no problems except that it only works with one-digit numbers. For example: 10+2 evaluates to 2 instead of 12. Here's the code of the parser:

package calculator;

public class ParsePost {

    private StackX theStack;
    private String input;

    public ParsePost(String s) {
        input = s;
    }

    public int doParse() {
        theStack = new StackX(20);
        char ch;
        int j;
        int num1, num2, interAns;
        for (j = 0; j < input.length(); j++){
            ch = input.charAt(j);
            if (ch >= '0' && ch <= '9'){
                theStack.push((int) (ch - '0'));
            }
            else{
                num2 = theStack.pop();
                num1 = theStack.pop();
                switch (ch){
                    case '+':
                        interAns = num1 + num2;
                        break;
                    case '-':
                        interAns = num1 - num2;
                        break;
                    case '*':
                        interAns = num1 * num2;
                        break;
                    case '/':
                        interAns = num1 / num2;
                        break;
                    default:
                        interAns = 0;
                }
                theStack.push(interAns);
            }
        }
        interAns = theStack.pop();
        return interAns;
    }
}

The program uses stack. The code is:

public class StackX {

        private int maxSize;
        private char[] stackArray;
        private int top;

        public StackX(int s){
            maxSize = s;
            stackArray = new char[maxSize];
            top = -1;
        }
        
        public void push(char j){
            stackArray[++top] = j;
        }

        public void push(int j) {
            stackArray[++top] = (char) j;
        }

        public char pop(){
            return stackArray[top--];
        }

        public char peek() {
            return stackArray[top];
        }

        public boolean isEmpty() {
            return (top == -1);
        }
        public boolean isFull() {
            return (top == maxSize - 1);
        }
        
        public int size(){
            return top + 1;
        }

        public char peekN(int n){
            return stackArray[n];
        }

        public void displayStack(String s) {
            System.out.print(s);
            System.out.print("Stack (bottom-->top): ");
            for (int j = 0; j < size(); j++) {
                System.out.print(peekN(j));
                System.out.print(' ');
            }

            System.out.println("");
        }

    }

Please help me!

[javaAddict]

It is because you enter to the stack character by character:

theStack.push((int) (ch - '0'));

Meaning that this input: 10+2, will be entered in the stack like this:
1, 0, +, 2
You want this:
10, +, 2

Try this:

String s = "10+2-3/4*11";
StringTokenizer st = new StringTokenizer(s,"[+-/*]",true);

while (st.hasMoreTokens()) {
    System.out.println(st.nextToken());
}

And see what it prints.

Also for the above you will need you stack to take String array, not char:

String num2 = theStack.pop();
String num1 = theStack.pop();
if ("+".equals(operator)) {
     interAns = Integer.parseInt(num1) + Integer.parseInt(num2);
}